1. Field of the Invention
The invention relates to the forming of metal under applied pressure and heat in dies and more particularly to superplastic forming of metal sheets in forming dies.
2. Description of the Related Art
In the manufacture of complex metal shapes from sheet metal, it is common to utilize die that have internal surfaces patterned in accordance with the desired shape to be imparted to the sheet metal. Certain metals, under applied heat and pressure, have superplastic properties that allow the metal to be deep drawn for forming into complex shapes. Sheets of these metals also diffusion bond to adjacent sheets under applied heat and pressure in the die through commingling of atoms of one sheet with the other resulting in a unitary structure.
Several techniques are used in superplastic forming and diffusion bonding. For example, to prevent “sticking” or bonding of metal sheets to a die surface, certain chemical release agents may be applied such as Yttrium Oxide. When sheets are stacked into a pack for superplastic forming and diffusion bonding into a stiffened structure, it is common to weld several gas inlet tubes between sheets of the pack.
One of the limitations of the superplastic forming technology is the tendency for the formed structure to suffer from “surface mark-off.” Surface mark-off is a marring (often in the form of a crease) of the surface of the formed metal structure. Typically mark offs appear on an opposite side to the patterned side. For example, when the forming process requires a deep draw on the metal when a stiffener is being formed, the mark off appears as crease on the other side of the stiffener. There is a need to develop a superplastic die forming method and/or apparatus that reduces or eliminates mark-off.